1. Field of the Invention
This invention relates to a heating device intended as a heat source for a fixing unit to be used in electrophotographic copying systems, printers, and facsimile systems. More specifically, this invention relates to a heating device which makes use of induction heating.
2. Description of the Related Art
The electrophotographic copying system, for example, is provided with a fixing unit which fixes on the sheet of a recording paper or a transfer material a developing agent which has been transferred to the sheet. As the heat source for this fixing unit, what makes use of electromagnetic induction heating has been proposed.
The fixing unit which utilizes induction heating is at an advantage in allowing faster elevation of temperature and consuming less electric power than the unit which effects indirect heating by the use of a halogen lamp as a heat source.
Generally, the control of the temperature of the fixing unit is attained by a procedure which comprises detecting the temperature of a fixing roller by the use of a thermistor kept in contact with the fixing roller in rotation, comparing the detected temperature with a set temperature (in the approximate range of 150-200.degree. C., for example), and effecting ON-OFF control of the heat source or duty control based on the outcome of the comparison. The control of this nature has found utility not only in the fixing unit using a halogen lamp as the heat source but also in the fixing unit resorting to induction heating.
The thermistor which is a temperature detecting means generally produces a delay in such a feedback control as is described above because of its own thermal capacity and delay in heat transmission. The delay has posed no particular problem to the fixing unit which uses a halogen lamp allowing no noticeably fast temperature elevation as the heat source. In the fixing unit which utilizes induction heating capable of high-speed temperature elevation, the response characteristic of the thermistor affects the temperature ripple and the overshoot during the control possibly to a point where the temperature of the roller of the fixing unit will not be retained constantly and the quality of the fixed image will be adversely affected.
A method for predicting a heating condition by detecting the upward or downward trend of temperature has been conceived. It does not easily obtain a stable fixing property. For this method to attain the stable fixing property, it incurs such problems as necessitating the development of a control device exclusively used for that purpose and inevitably increasing the cost of equipment.
As a fixing unit of the form utilizing the aforementioned induction heating, JP-A-07-114,276, for example, discloses a fixing unit which attains the fixation of a toner image on a recording paper by causing a film possessing a conducting layer in conjunction with the recording paper carrying thereon an unfixed toner image to be nipped in a nipping part between a supporting member and a pressing roller and enabling the pressing roller to advance sympathetically the film in conjunction with the recording paper.
The fixing unit of this form, however, has the problem that when the film possessing the conducting layer happens to sustain a scratch for some cause or other while it is verging on rotation, this film is seriously fractured in consequence of the aggravation of the scratch by the rotation. When the film suffers the fracture of this nature, the fractured film will possibly scatter in the fixing unit and inflict damage to even the parts other than the fixing unit. If the fractured film falls short of being scattered, there still arises the problem that the magnetic flux which is inherently concealed by the conducting layer of the film possibly leaks through the scraped part and exposes the other metallic members to induction heating.